Means for electric vacuum furnaces



Jan. 16, 1962 AGE HANSEN MEANS FOR ELECTRIC VACUUM FURNACES Filed Nov.5, 1958 p INVENTOR.

Age Hansen United States Patent O MEANS FOR ELECTRIC VACUUM FURNACES AgeHansen, Vasteras, Sweden, assignor to Allmanna Svenska ElektriskaAktieholaget, Vasteras, Sweden, a Swedish corporation Filed Nov. 5,1958, Ser. No. 772,133 Claims priority, application Sweden Nov. 19, 19572 Claims. (Cl. 219-4059) In an electric vacuum furnace, for example ahigh frequency furnace, corona arises on the live parts of the furnaceon its coil or on the connections to the coil, due to the low pressureof the gas which is present in the space in which the furnace isaccommodated. This corona may directly damage the parts on which thecorona occurs, but it is also indirectly detrimental due to thegeneration of ozone Which damages by oxidation, both the metal parts andthe insulation material.

This corona may be avoided according to the present invention, in thatthe induction coil and other live parts are coated with a layer ofinsulating material which is provided on its outer side with aconductive coating, suitably of non-magnetic material with comparativelyhigh conduction resistance. By this measure the voltage stresses areconcentrated into the insulation coating so that the low pressure gaswhich surrounds the live parts is entirely freed of electric stresses.

To avoid corona it is however necessary for the live parts to beentirely shielded by the conductive layer, which need not be co-herentbut may consist of two or more parts which are not joined together butover-lap each other electrostatically.

The arrangement has to be such that no heavy currents can arise in theconductive coating by contact between the live parts having differentpotential or by electric voltages. Since, on the other hand, acomparatively good conductivity of the conductive coating is desired theshield, as a rule, has to be divided into separate parts but theinterstices between these parts have to be bridged by a conductivecoating which is insulated from parts of the shield. Generally speakingthe live parts have to be shielded from the gas within the vacuumcontainer by means of an electrostatically tight shield. Such a tightshielding may be obtained either by coating the live parts with aninsulating lacquer layer which is coated on the outside with a thinconductive layer of a so called corona protecting lacquer suitablyhaving a high resistance. Instead of coating the live parts with theinsulating lacquer layer which is coated on the outside with a more orless conducting layer, live bodies of simple shape may be insulated bywinding a strip of insulating material around them, which is provided onone side with a conducting layer, for example, metal foil; this foilshould have a width which is less than the width of the strip, so thatthe metal foil in one turn, does not come into contact with the metalfoil on the succeeding turn of the strip.

The accompanying drawing shows, in FIG. 1, a vertical view of aninduction coil in an electric furnace which is assumed to be enclosed ina vacuum container not shown on the drawing. FIG. 2 shows the same induction coil from above. FIG. 3 is a cross-section of a conductor. FIG. 4is a section through the winding along the line A-A in FIG. 2, whileFIG. 5 illustrates an em- 3,017,485 Patented Jan. 16, 1962 "ice bodimenthaving a layer of insulating material on the conductor.

On the drawing 1 designates generally the turns of the coil, while 2designates its connection leads. 3 is a part of the wall of thecontainer surrounding the coil and 4 shows diametrically an insulatingsealing gland which is attached to the wall of the vacuum containeraround an opening in which the leads 2 pass through the sealing gland.

In FIG. 3, a section of a conductor is shown, by way of example, whichforms the furnace coil. 5 is the tubeshaped copper conductor which issurrounded by a sleeve of insulating material 6 which is in turnsurrounded by a conducting shield 7. This shield 7 is connected, as canbe seen from FIG. l, by conductors 8, to earth or another suitablepotential. The sleeve and the surrounding shield may consist of a strip6' of insulating material which is coated on one side with a conductinglayer 7 which terminates short of both edges of the insulating strip 6'.

FIGURE 2 shows an embodiment of the invention wherein the conductivecoating 7 is divided into parts along each turn of its periphery so thatgaps or interstices are formed, indicated by broken lines. The gaps arecovered by layers 9 of insulating material and these layers are partlycovered by metal strips 10, preferably so that one rim of the metalstrip is in contact with the underlying part of the conductive coating 7while the other rim of the metal strip 10 is insulated from theunderlying part of the conductive coating. See also the section 5.

I claim as my invention:

l. Means for avoiding corona of the induction coil and its associatedlive parts in a high-frequency induction furnace enclosed in an emptyspace, comprising an insulating layer on the conductor of the coil andits associated live parts, a conductive coating of comparatively highelectric resistivity on said insulating layer divided into a pluralityof separate parts along the turns of the coil, solid insulating layerscovering the interstices between the conductive coating parts andcontacting the solid conductive coating therebetween, a second layer ofconductive material overlying each of the said insulating layer, saidsecond layer being in contact with the end of one underlying conductivecoating and the insulating layer and being insulated from the adjacentbeginning of the other underlying conductive coating and leadsconnecting the said second layers of conductive material to a potentialsubstantially lower than the potential of the coil and its associatedlive parts.

2. Means according to claim 1, characterised in that the insulatinglayer and the conductive coating are made by means of strips ofinsulating material which are provided on one side with a conductingcoating having less Width than the strip.

References Cited in the file of this patent UNITED STATES PATENTS

